Low resistance ultracapacitor electrode and manufacturing method thereof

ABSTRACT

A carbon-based electrode includes activated carbon, carbon black, and a binder. The binder is fluoropolymer having a molecular weight of at least 500,000 and a fluorine content of 40 to 70 wt. %. A method of forming the carbon-based electrode includes providing a binder-less conductive carbon-coated current collector, pre-treating the carbon coating with a sodium napthalenide-based solution, and depositing onto the treated carbon coating a slurry containing activated carbon, carbon black and binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Application Ser. No. 61/872,192 filed on Aug. 30, 2013, the entirecontent of which is hereby incorporated by reference.

BACKGROUND

1. Field

The present disclosure relates generally to carbon-based electrodes forenergy storage devices, and more specifically to low resistanceelectrodes that include a high molecular weight fluoropolymer binder andtheir methods of production.

2. Technical Background

Energy storage devices such as ultracapacitors may be used in a varietyof applications such as where a discrete power pulse is required.Example applications range from cell phones to hybrid vehicles.Ultracapacitors also known as electrochemical double layer capacitors(EDLCs) have emerged as an alternative or compliment to batteries inapplications that require high power, long shelf life, and/or long cyclelife. Ultracapacitors typically comprise a porous separator and anorganic electrolyte sandwiched between a pair of carbon-basedelectrodes. The energy storage is achieved by separating and storingelectrical charge in the electrochemical double layers that are createdat the interfaces between the electrodes and the electrolyte. Importantcharacteristics of these devices are the energy density and powerdensity that they can provide, which are both largely determined by theproperties of the carbon that is incorporated into the electrodes.

BRIEF SUMMARY

In accordance with embodiments of the present disclosure, carbon-basedelectrodes such as for incorporation into ultracapacitors or other highpower density energy storage devices, include a carbon mat comprisingactivated carbon, carbon black and a binder. The carbon mat is disposedadjacent to a current collector. The binder can comprise a highmolecular weight fluoropolymer having, for example, 40-70 wt. %fluorine. High molecular weight polymers can have a molecular weight ofat least 500,000. An example fluoropolymer is Kynar® grade PVDF.

In further related embodiments, a high-purity, thermally-grown carbonlayer can be used as an alternative to conductive ink as a conductivelayer between the carbon mat and the current collector. Athermally-grown carbon layer is free of a binder. Devices with abinder-free conductive carbon layer have an ESR that is less thansimilar devices where such a layer is formed from commercially-availableconductive inks.

A method for forming carbon-based electrodes includes pre-treating thebinder-less (thermally-grown carbon) conductive layer with a sodiumnapthalenide-based solution. The solution improves adhesion between thecarbon mat and the current collector.

Additional features and advantages of the subject matter of the presentdisclosure will be set forth in the detailed description which follows,and in part will be readily apparent to those skilled in the art fromthat description or recognized by practicing the subject matter of thepresent disclosure as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the subjectmatter of the present disclosure, and are intended to provide anoverview or framework for understanding the nature and character of thesubject matter of the present disclosure as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe subject matter of the present disclosure, and are incorporated intoand constitute a part of this specification. The drawings illustratevarious embodiments of the subject matter of the present disclosure andtogether with the description serve to explain the principles andoperations of the subject matter of the present disclosure.Additionally, the drawings and descriptions are meant to be merelyillustrative, and are not intended to limit the scope of the claims inany manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a plot of differentiated current versus potential with respectto Ag/AgCl reference electrode for PTFE and PVDF-containing carbon-basedelectrodes;

FIG. 2 shows a plan-view SEM micrograph of a thermally-grown carbonlayer;

FIG. 3 shows a cross-sectional SEM micrograph of a thermally-growncarbon layer on an aluminum current collector;

FIG. 4 is a schematic representation of the reaction between sodiumnapthalenide and PVDF;

FIG. 5 is a schematic illustration of an example ultracapacitor;

FIG. 6 shows Nyquist plots for coin cells comprising carbon-basedelectrodes at 0V;

FIG. 7 shows Nyquist plots for coin cells comprising carbon-basedelectrodes at 2.7V; and

FIG. 8 shows Nyquist plots for coin cells comprising carbon-basedelectrodes at 3V.

DETAILED DESCRIPTION

Reference will now be made in greater detail to various embodiments ofthe subject matter of the present disclosure, some embodiments of whichare illustrated in the accompanying drawings. The same referencenumerals will be used throughout the drawings to refer to the same orsimilar parts.

Carbon-based electrodes suitable for incorporation into energy storagedevices are known. Activated carbon is widely used as a porous materialin ultracapacitors due to its large surface area, electronicconductivity, ionic capacitance, chemical stability, and/or low cost.Activated carbon can be made from synthetic precursor materials such asphenolic resins, or natural precursor materials such as coals orbiomass. With both synthetic and natural precursors, the activatedcarbon can be formed by first carbonizing the precursor and thenactivating the intermediate product. The activation can comprisephysical (e.g., steam) or chemical activation (e.g., KOH) at elevatedtemperatures to increase the porosity and hence the surface area of thecarbon. The carbon-based electrodes can include, in addition toactivated carbon, a conductive carbon such as carbon black, and a bindersuch as polytetrafluoroethylene (PTFE). The activated carbon-containinglayer (carbon mat) is typically laminated over a current collector toform the carbon-based electrode.

The choice of separator and electrode materials directly affect theperformance of the device, including the achievable energy density andpower density. The energy density (E) of an EDLC is given by E=½ CV²,and power density (P) of an EDCL is given by P=V²/R, where C is thecapacitance, V is the device's operating voltage, and R is theequivalent series resistance (ESR) of the device.

The ESR has both electronic and an ionic components. The former includesresistance from the carbon-based electrode, including interfacialresistance between the carbon mat and the current collector as well ascell package resistance. The latter is related to the conductivity ofelectrolyte, and interactions between the electrolyte and the porouscarbon.

Recently, with a goal of increasing the energy density and power densityof EDLC devices, engineered carbon materials have been developed toachieve higher capacitance. To achieve higher capacitance, activatedcarbon materials with high surface area (500-2500 m²/g) may be used.

A further approach to increasing the energy density and power density isto increase the capacitor's operating voltage. In this regard, aqueouselectrolytes have been used in EDLCs for lower voltage (<1V) operation,while organic electrolytes have been used for higher voltage (2.3-2.7 V)devices. However, to achieve even higher energy densities, there is aneed to increase the voltage envelop from conventional values of about2.7 V to around 3.0 V. Such an increase from 2.7 to 3.0 V will result ina 23% increase in the energy density. A still further approach toincreasing the power density is to minimize the capacitor's ESR.

Thus, in order to realize higher energy densities and higher powerdensities, next generation EDLCs will likely operate at high appliedvoltages. As a consequence, it may be desirable to minimize unwantedFaradaic reactions between the binder and the liquid electrolyte,particularly at the higher potentials and reduce the device's ESR byoptimizing the interfacial resistance between the carbon mat and thecurrent collector.

In various embodiments, a carbon-based electrode includes activatedcarbon, carbon black and a binder. The carbon-based electrode caninclude 75-90 wt. % activated carbon, 5-10 wt. % carbon black, and 5-15wt. % binder. It has been shown that the choice of binder may influencethe stability of the electrode when incorporated into an EDLC,particularly at operating voltages greater than 2.7V. Suitable bindermaterials for forming the carbon-based electrode are high molecularweight fluoropolymers.

High molecular weight fluoropolymer binder materials can have amolecular weight of at least 500,000, e.g., at least 800,000, and cancomprise from 40 to 70 wt. % fluorine (e.g., from 50-70 wt. % fluorine).An example fluoropolymer is polyvinylidene fluoride (PVDF).

PVDF has a glass transition temperature (T_(g)) of about −35° C. and istypically 50-60% crystalline. PVDF may be synthesized from the gaseousVDF monomer via a free radical (or controlled radical) polymerizationprocess. PVDF is marketed under a variety of brand names including Hylar(Solvay), Kynar (Arkema) and Solef (Solvay).

In embodiments, the PVDF incorporated as a binder into a carbon-basedelectrode has a molecular weight of at least 500,000 (e.g., at least800,000). PVDF is an alternative to PTFE, which is widely used as abinder material in carbon-based electrodes. PVDF-containing electrodeshave been shown to be more stable than PTFE-containing electrodes inexample EDLC devices, however, particularly at operating voltagesgreater than 2.7V (i.e., greater than 3V).

The stability of negative electrodes comprising different binders wasevaluated using a three electrode setup, which involved polarizing theelectrodes to extreme potentials with respect to a Ag/AgCl referenceelectrode. As shown in FIG. 1, the electrode comprising a comparativePTFE binder (curve A) exhibits a high reduction current at about −1.8V.Without wishing to be bound by theory, this current is believed to bedue to reductive decomposition of PTFE through de-fluorination. PTFEde-fluorination is believed to weaken the negative electrode matrix andlead to electrode embrittlement. Such embrittlement has been observedexperimentally. It is also believed that fluorine from PTFE may reactwith trace moisture and generate unwanted HF acid within the cell.Additionally, the highly irreversible reduction reaction causes thepotential of the positive electrode to be shifted unfavorably into aregion of irreversible oxidation, further degrading the cell.

In contrast, still referring to FIG. 1, the negative electrodecomprising PVDF binder (curve B) exhibits a lesser reduction current atabout −1.8V, consistent with the conclusion that PVDF is less prone tovoltage-induced degradation. Without wishing to be bound by theory, thelower relative fluorine content of PVDF may account for its enhancedhigh-voltage stability compared to PTFE. PVDF, which has the generalizedformula (CH₂CF₂)_(n), includes half the fluorine of PTFE, which has thestructural unit (C₂F₄)_(n). Compositionally, PVDF is about 59 wt. %fluorine, while PTFE is about 76 wt. % fluorine. A further examplefluoropolymer is (CHFCF₂)_(n), which is about 69.5 wt. % fluorine.

Various methods can be used to form a carbon-based electrode (e.g.,carbon mat) that includes a high molecular weight fluoropolymer binder.One method of making a carbon-based electrode includes forming a slurrycomprising activated carbon, carbon black, binder and a liquid carrier.A carbon mat can be formed by coating a substrate with the slurry anddrying the coating to remove the liquid carrier.

The liquid carrier may be isopropyl alcohol, n-methyl pyrrolidone (NMP),dimethyl formamide (DMF), dimethyl acetamide (DMAc), etc. which canfacilitate adhesion of the component particles during processing as wellas the formation of a thin film of the components via casting. PVDF, forexample, is soluble in NMP. A slurry comprising activated carbon, carbonblack and PVDF binder in NMP can therefore include solid particles ofthe activated carbon and carbon black, while the PVDF will be insolution until the solvent is removed.

In various embodiments, a slurry comprising activated carbon, carbonblack, binder and a liquid carrier or liquid solvent can be deposited(e.g., slot coated) onto a substrate to form a thin film that is driedto produce a carbon mat. The thin film can be dried, for example, in aconventional oven or in a vacuum oven. The substrate may be a currentcollector such that the carbon-based electrode is formed in situ. Thecurrent collector may include a conductive carbon layer onto which theslurry is deposited.

To improve the mechanical integrity of the carbon mat/current collectorcomposite, the deposited thin film can be laminated onto the currentcollector, which compacts the layers. The application of pressure may beperformed at elevated temperature, e.g., about 200° C. The laminatedelectrodes may be cut to the appropriate dimensions and wound into ajelly roll together with cellulosic separator paper (NKK TF4030). Thecurrent collector ends are smeared and laser welded to terminals. Theassembly is then packaged into an aluminum can and sealed. The resultingcell is dried in vacuum at 130° C. for 48 hrs. Electrolyte is filledinto the cell, and the cell is sealed.

In embodiments, the carbon mat is laminated onto one or both sides of aconductive current collector. The current collector can be, for example,a 15-40 μm (e.g., 20 micron) thick aluminum foil that is optionallypre-coated with a layer of conductive carbon such as thermally-growncarbon. With respect to commercially-available conductive inks,thermally-grown carbon may contain fewer transition metal contaminants,which can aid in minimizing unwanted Faradic reactions and reducing theESR. The thermally-grown carbon layer, which is free of a binder, mayalso promote a low ESR through the current collector via the formationof electrically conductive aluminum carbide (Al₄C₃) particles at theinterface between the carbon layer and the aluminum. The layer ofconductive carbon can be free or substantially free of organics, suchthat, in embodiments, the organic content of the conductive carbon layeris less than 1 wt. % or less than 0.5 wt. %. For example, the organiccontent of the conductive carbon layer can range from 100 ppm to 10000ppm, e.g., 100, 200, 500, 1000, 5000 or 10000 ppm, including rangesbetween any of the foregoing values.

Scanning electron microscope (SEM) micrographs of a thermally-growncarbon layer 53 disposed over an aluminum current collector 55 are shownin FIG. 2 (plan view) and FIG. 3 (cross-sectional view).

Notwithstanding a step of lamination, adhesion of the carbon mat to thecurrent collector can be improved by pre-treating the current collectorsurface. Such a pre-treatment may include etching the surface (e.g.,etching the binder-less carbon surface) prior to applying or forming thecarbon mat. Another approach includes applying an etchant to the carbonmat.

The use of an etchant significantly improves the adhesion of the carbonmat with the binder-less carbon-coated aluminum current collector. Oneexample etchant is a solution of sodium napthalenide in 2-methoxyethylether, though other alkali metal napthalenides can be used. For example,further etchants include lithium napthalenide and potassiumnapthalenide.

Fluoropolymer binders such PVDF are made up of carbon atoms, hydrogenatoms and fluorine atoms. A sodium napthalenide-based etchant containsmetallic sodium in solution. The sodium reacts with the fluorine of thefluoropolymer, extracting it, which leaves the molecule unbalanced.During subsequent exposure to ambient conditions, hydrogen and oxygenatoms restore the equilibrium of the molecule. This results in acarbonaceous backbone rich in functional groups that are responsible foradhesion.

A schematic representation of the reaction between sodium napthalenideand PVDF is shown in FIG. 4. The by-products of the reaction are sodiumfluoride (NaF) and naphthalene.

In an example method, an etchant is prepared for conditioning thebinder-less carbon coated current collector. Sodium napthalenidesolution in 2-methoxyethyl ether is the base material for the process.Such a solution may be obtained at a concentration of 10-30 wt. % sodiumnapthalenide, which may be further diluted, for example, by the additionof tetrahydrofuran (THF) to yield a 2-5 wt. % solution of sodiumnapthalenide/2-methoxyethyl ether in THF.

The etchant solution can be coated onto the binder-less carbon coatedcurrent collector using a variety of methods such as spray coating, slotcoating or gravure roll coating. The sodium napthalenide solution can beallowed to dry. Electrode slurry comprising activated carbon, carbonblack and binder can in turn be coated onto the treated binder-lesscarbon coated current collector, dried and passed through a pair oflamination rollers to form a low ESR carbon-based electrode.

The present disclosure also relates to an energy storage device, such asan electrochemical double layer capacitor (EDLC), comprising at leastone carbon-based electrode that includes the high molecular weight PVDFbinder material described herein. Such a device can also include abinder-less conductive carbon layer within the carbon-based electrode,i.e., at the interface between the carbon mat and the aluminum currentcollector.

Ultracapacitors typically comprise two porous electrodes that areisolated from electrical contact with each other by a porous dielectricseparator. The separator and the electrodes are impregnated with anelectrolytic solution that allows ionic current to flow between theelectrodes while preventing electronic current from discharging thecell. Each porous electrode is typically in electrical contact with acurrent collector. The current collector, which can comprise a sheet orplate of electrically-conductive material (e.g., aluminum) can reduceohmic losses while providing physical support for the porous electrode(activated carbon) material, i.e., the carbon mat.

According to embodiments, an electrochemical cell comprises a firstcarbon-based electrode and a second carbon-based electrode arrangedwithin a casing, wherein each carbon-based electrode includes a currentcollector having opposing first and second major surfaces, a firstconductive layer is disposed adjacent to the first major surface, asecond conductive layer is disposed adjacent to the second majorsurface, and a first carbon-based layer and a second carbon-based layereach comprising activated carbon, carbon black and binder are disposedadjacent to respective ones of the first and second conductive layers.One or both of the conductive layers can comprise a thermally-grown(binder-less) carbon layer.

FIG. 5 is a schematic illustration of an example ultracapacitor.Ultracapacitor 10 includes an enclosing body 12, a pair of currentcollectors 22, 24, a first carbon mat 14 and a second carbon mat 16 eachrespectively disposed adjacent to one of the current collectors, and aporous separator layer 18. Electrical leads 26, 28 can be connected torespective current collectors 22, 24 to provide electrical contact to anexternal device. Layers 14, 16 may comprise activated carbon, carbonblack and high molecular weight fluoropolymer binder. A liquidelectrolyte 20 is contained within the enclosing body and incorporatedthroughout the porosity of both the porous separator layer and each ofthe porous electrodes. In embodiments, individual ultracapacitor cellscan be stacked (e.g., in series) to increase the overall operatingvoltage.

The enclosing body 12 can be any known enclosure means commonly-usedwith ultracapacitors. The current collectors 22, 24 generally comprisean electrically-conductive material such as a metal, and commonly aremade of aluminum due to its electrical conductivity and relative cost.For example, current collectors 22, 24 may be thin sheets of aluminumfoil.

Porous separator 18 electronically insulates the electrodes from eachother while allowing ion diffusion. The porous separator can be made ofa dielectric material such as cellulosic materials, glass, and inorganicor organic polymers such as polypropylene, polyesters or polyolefins. Inembodiments, a thickness of the separator layer can range from about 10to 250 microns.

The electrolyte 20 serves as a promoter of ion conductivity, as a sourceof ions, and may serve as a binder for the carbon. The electrolytetypically comprises a salt dissolved in a suitable solvent. Suitableelectrolyte salts include quaternary ammonium salts such as thosedisclosed in commonly-owned U.S. patent application Ser. No. 13/682,211,the disclosure of which is incorporated herein by reference. Examplequaternary ammonium salts include tetraethylammonium tetraflouroborate((Et)₄NBF₄) or triethylmethyl ammonium tetraflouroborate (Me(Et)₃NBF₄).

Example solvents for the electrolyte include but are not limited tonitriles such as acetonitrile, acrylonitrile and propionitrile;sulfoxides such as dimethyl, diethyl, ethyl methyl and benzylmethylsulfoxide; amides such as dimethyl formamide and pyrrolidones such asN-methylpyrrolidone. In embodiments, the electrolyte includes a polaraprotic organic solvent such as a cyclic ester, chain carbonate, cycliccarbonate, chain ether and/or cyclic ether solvent. Example cyclicesters and chain carbonates have from 3 to 8 carbon atoms, and in thecase of the cyclic esters include β-butyro-lactone, γ-butyrolactone,γ-valerolactone and δ-valerolactone. Examples of the chain carbonatesinclude dimethyl carbonate, diethyl carbonate, dipropyl carbonate,ethylene carbonate, methyl ethyl carbonate, methyl propyl carbonate andethyl propyl carbonate. Cyclic carbonates can have from 5 to 8 carbonatoms, and examples include 1,2-butylene carbonate, 2,3-butylenecarbonate, 1,2-pentene carbonate, 2,3-pentene carbonate and propylenecarbonate. Chain ethers can have 4 to 8 carbon atoms. Example chainethers include dimethoxyethane, diethoxyethane, methoxyethoxyethane,dibutoxyethane, dimethoxypropane, diethoxypropane andmethoxyethoxypropnane. Cyclic ethers can have from 3 to 8 carbon atoms.Example cyclic ethers include tetrahydofuran, 2-methyl-tetrahydrofuran,1,3-dioxolan, 1,2-dioxolan, 2-methyldioxolan and 4-methyldioxolan. Acombination of two or more solvents may also be used.

As examples, an assembled EDLC can comprise an organic liquidelectrolyte such as tetraethylammonium tetrafluoroborate (TEA-TFB) ortriethylmethylammonium tetrafluoroborate (TEMA-TFB) dissolved in anaprotic solvent such as acetonitrile.

Ultracapacitors may have a jelly roll design, prismatic design,honeycomb design, or other suitable configuration. A carbon-basedelectrode made according to the present disclosure can be incorporatedinto a carbon-carbon ultracapacitor or into a hybrid ultracapacitor. Ina carbon-carbon ultracapacitor, both of the electrodes are carbon-basedelectrodes. In a hybrid ultracapacitor, one of the electrodes iscarbon-based, and the other electrode can be a pseudo capacitivematerial such as lead oxide, ruthenium oxide, nickel hydroxide, oranother material such as a conductive polymer (e.g.,parafluorophenyl-thiophene).

In carbon-carbon ultracapacitors, the activated carbon in each electrodemay have the same, similar or distinct properties. For example, the poresize distribution of the activated carbon incorporated into a positiveelectrode may be different than the pore size distribution of theactivated carbon incorporated into a negative electrode.

The activated carbon used in conjunction with the carbon-basedelectrodes disclosed herein can have a specific surface area greaterthan about 300 m²/g, i.e., greater than 350, 400, 500 or 1000 m²/g. Inembodiments, the average particle size of the activated carbon can bemilled to less than 20 microns, e.g., about 5 microns, prior toincorporating the activated carbon into a carbon-based electrode.

Within an individual ultracapacitor cell, and under the influence of anapplied electric potential, an ionic current flows due to the attractionof anions in the electrolyte to the positive electrode and cations tothe negative electrode. Ionic charge can accumulate at each of theelectrode surfaces to create charge layers at the solid-liquidinterfaces. The accumulated charge is held at the respective interfacesby opposite charges in the solid electrode to generate an electrodepotential.

During discharge of the cell, a potential across the electrodes causesionic current to flow as anions are discharged from the surface of thepositive electrode and cations are discharged from the surface of thenegative electrode. Simultaneously, an electronic current can flowthrough an external circuit located between the current collectors. Theexternal circuit can be used to power electrical devices.

The amount of charge stored in the layers impacts the achievable energydensity and power density of the capacitor. The performance (energy andpower density) of an ultracapacitor depends largely on the properties ofthe activated carbon that makes up the electrodes. The properties of theactivated carbon, in turn, can be gauged by evaluating, for example, theporosity and pore size distribution of the activated carbon, as well asthe impurity content within the activated carbon, such as nitrogen oroxygen. Relevant electrical properties include the potential window,area-specific resistance and the volumetric capacitance.

The disclosed ultracapacitors may, in some embodiments, exhibitoperating voltages up to 3.2 V (e.g., 2.7, 2.8, 2.9, 3.0, 3.1 or 3.2V)and a volumetric capacitance of greater than 50 F/cm³ (e.g., greaterthan 50, 60, 70, or 80 F/cm³), including capacitance values between anyof the foregoing values. The high potential window is believed to be theresult of the high purity conductive carbon layer and/or the lowreactivity of the binder, e.g., PVDF.

Various embodiments will be further clarified by the following examples.

EXAMPLES Example 1 PVDF Electrode+Binder-Less Carbon Conductive Layer

Carbon-based electrodes were fabricated with Kynar® 761 PVDF homopolymeras the binder. The molecular weight of the 761 PVDF is in the range of300,000 to 400,000. The electrodes were cast onto a current collectorsubstrate using a slurry method.

A dry mixture of the carbon-based electrode constituents was initiallyprepared, including 90 wt. % activated carbon, 5 wt. % carbon black(Cabot BP2000), and 5% Kynar® 761 PVDF. The activated carbon was achemically-activated carbon derived from wheat flour. The solid mixturewas ball milled for 30 min at 350 rpm.

NMP solvent was added to the ball-milled mixture and the resultingslurry was ball-milled again for 30 minutes at 350 rpm. The slurry wasapplied using a doctor blade directly onto a 20 micron thick aluminumcurrent collector provided with a binder-less carbon conductive layer(TOYO Corporation, Tokyo Japan). The carbon mat thickness was about 100μm.

The coated current collector was dried at 140-150° C. under vacuum, andthen laminated at 140-150° C. to obtain the carbon-based electrode.Adhesion of the carbon mat to the current collector was evaluated usinga tape test. The results indicated poor adhesion of the carbon mat tothe current collector.

Example 2 PVDF Electrode+Binder-Less Carbon Conductive Layer

Carbon-based electrodes were fabricated with Kynar® 301F PVDFhomopolymer as the binder using the process described in Example 1. Themolecular weight of the 301F PVDF is in the range of 500,000 to 700,000.Tape test results indicated moderate adhesion of the carbon mat to thecurrent collector.

Example 3 PVDF Electrode+Binder-Less Carbon Conductive Layer

Carbon-based electrodes were fabricated with Kynar® HSV 900 PVDFhomopolymer as the binder using the process described in Example 1,except that the thickness of the carbon mat was about 133 μm. Themolecular weight of the HSV 900 PVDF is about 1,000,000. Tape testresults indicated moderate adhesion of the carbon mat to the currentcollector.

Examples 1-3 demonstrate that by increasing the molecular weight of thePVDF polymer to at least 500,000, which corresponds to effectivelydecreasing the glass transition temperature (T_(g)) of the binder, amore mechanically robust carbon-based electrode can be produced. Inembodiments, the molecular weight of the PVDF polymer is at least800,000. Such electrodes are well-suited to forming EDLCs have ajelly-roll design. A summary of the results from Examples 1-3 are shownin Table 1.

TABLE 1 Carbon-based electrodes with different grades of PVDF binder Ex.# PVDF MW t (microns) Adhesion 1 Kynar ® 761 300,000-400,000 100 Poor 2Kynar ® 301F 500,000-700,000 100 Moderate 3 Kynar ® HSV900 Approx.1,000,000 133 Moderate

Example 4 PTFE Electrode+Ink-Coated Conductive Layer

Comparative carbon-based electrodes were fabricated on conductive carbonink-coated aluminum foil current collectors using PTFE as the binder.

A solid mixture of 85 wt. % activated carbon, 5 wt. % carbon black, and10 wt. % PTFE binder (DuPont 601A) was ball-milled for 30 minutes at 350rpm, and then calendared to obtain a 105 micron thick free-standingcarbon mat.

The carbon mat was laminated onto an aluminum foil current collectorprovided with an ink-based (DAG EB012, Henkel), binder-containingconductive carbon coating. The DAG ink includes a vinyl pyrrolidonepolymer binder. Tape test results indicated good adhesion of the carbonmat to the current collector.

Dried electrodes were incorporated into coin cells for ESR measurements.The measured ESR was at 2.3Ω at 3.0 V. The high ESR value was attributedto the binder-containing conductive carbon layer.

Example 5 PVDF Electrode+Un-Etched Binder-Less Carbon Conductive Layer

Carbon-based electrodes were fabricated with Kynar® HSV 900 PVDFhomopolymer as the binder. The carbon mat was deposited using a slurrycasting method similar to that used in Example 1, but to achieve a 97 μmthick carbon mat. The slurry was applied using a doctor blade directlyonto a current collector provided with a binder-less, thermally-grownconductive layer (TOYO Corporation, Tokyo, Japan).

With respect to commercially-available conductive inks, thermally-growncarbon may contain fewer transition metal contaminants, which can aid inminimizing unwanted Faradic reactions.

Following lamination, tape test results indicated moderate adhesion ofthe carbon mat to the current collector. Dried electrodes wereincorporated into coin cells for ESR measurements. The measured ESR wasat 0.453Ω at 3.0 V.

Example 6 PVDF Electrode+1% Etched Binder-Less Carbon Conductive Layer

Carbon-based electrodes were fabricated with Kynar® HSV 900 PVDFhomopolymer as the binder. The carbon mat was deposited using a slurrycast method similar to that used in Example 1, but to achieve a 97 μmthick carbon mat.

Prior to lamination, the binder-less carbon coated current collector wasetched using a 1% solution of Na-napthalenide/2-methoxyethyl ether inTHF. The solution was spray coated onto the conductive carbon, which wasdried for 10-15 seconds.

Tape test was performed and complete delamination of the carbon layerfrom binder-less carbon coated current collector was observed. ESR datawas not obtainable for Example 6 due to the poor adhesion of the carbonmat.

Example 7 PVDF Electrode+2% Etched Binder-Less Carbon Conductive Layer

Carbon-based electrode were fabricated as in Example 6, except thebinder-less carbon coated current collector was etched using a 2%Na-napthalenide/2-methoxyethyl ether solution in THF.

Tape test results indicated good adhesion of the carbon mat to thecurrent collector. The measured ESR of the corresponding coin cells was0.735Ω at 3.0 V.

Example 8 PVDF Electrode+5% Etched Binder-Less Carbon Conductive Layer

Carbon-based electrode were fabricated as in Example 6, except thebinder-less carbon coated current collector was etched using a 5%Na-napthalenide/2-methoxyethyl ether solution in THF.

Tape test results indicated good adhesion of the carbon mat to thecurrent collector. The measured ESR of the corresponding coin cells was0.589Ω at 3.0 V.

Example 9 PVDF Electrode+7.5% Etched Binder-Less Carbon Conductive Layer

Carbon-based electrode were fabricated as in Example 6, except thebinder-less carbon coated current collector was etched using a 7.5%Na-napthalenide/2-methoxyethyl ether solution in THF.

Tape test results indicated good adhesion of the carbon mat to thecurrent collector. The measured ESR of the corresponding coin cells was1.042Ω at 3.0 V.

Examples 5-9 examine the effects of pre-treating a thermally-grownconductive carbon layer with sodium napthalenide-based etchant prior tolamination of the current collector with a PVDF-based carbon mat.

A summary of the results from Examples 4-9 are shown in Table 2. The ESRvalues are reported in Ohms (a).

Ex. # Cell Etchant Adhesion ESR@0 V ESR@2.7 V ESR@3 V 4 10% PTFE + inkn/a Good 0.910 1.814 2.328 5 5% PVDF + n/a Moderate 0.376 0.438 0.453thermal carbon 6 5% PVDF + 1% solution Poor n/a n/a n/a thermal carbon 75% PVDF + 2% solution Good 0.572 0.655 0.735 thermal carbon 8 5% PVDF +5% solution Good 0.528 0.535 0.589 thermal carbon 9 5% PVDF + 7.5% Good0.771 0.921 1.042 thermal carbon solution

ESR plots for Examples 4, 5 and 7-9 at 0V, 2.7V and 3V are shownrespectively in FIGS. 6-8.

In embodiments, a 1-10 wt. % (e.g., 2-5 wt. %) solution of sodiumnapthalenide/2-methoxyethyl ether may be used to pre-treat a currentcollector surface prior to forming a carbon mat on the surface.

Disclosed are carbon-based electrodes and associated methods for makingcarbon-based electrodes that can be incorporated into high energy, highpower performance EDLC devices. Various embodiments relate toincorporation of high molecular weight Kynar® grade polyvinylidene(PVDF) binder into the carbon mat. Further embodiments relate toproviding a binder-less conductive carbon coating (e.g., thermally-growncarbon) at the interface between the carbon mat and the currentcollector and the pre-treatment of such a carbon coating with sodiumnapthalenide/2-methoxyethyl ether to promote adhesion between the carbonmat and the carbon.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “binder” includes examples having two or moresuch “binder” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component being“configured” or “adapted to” function in a particular way. In thisrespect, such a component is “configured” or “adapted to” embody aparticular property, or function in a particular manner, where suchrecitations are structural recitations as opposed to recitations ofintended use. More specifically, the references herein to the manner inwhich a component is “configured” or “adapted to” denotes an existingphysical condition of the component and, as such, is to be taken as adefinite recitation of the structural characteristics of the component.

While various features, elements or steps of particular embodiments maybe disclosed using the transitional phrase “comprising,” it is to beunderstood that alternative embodiments, including those that may bedescribed using the transitional phrases “consisting” or “consistingessentially of,” are implied. Thus, for example, implied alternativeembodiments to an electrode that comprises activated carbon, carbonblack and a binder include embodiments where an electrode consists ofactivated carbon, carbon black and a binder and embodiments where ananode consists essentially of activated carbon, carbon black and abinder.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present disclosurewithout departing from the spirit and scope of the invention. Sincemodifications, combinations, sub-combinations and variations of thedisclosed embodiments may occur to persons skilled in the art, theinvention should be construed to include everything within the scope ofthe appended claims and their equivalents.

We claim:
 1. A carbon-based electrode comprising: a current collectorhaving opposing first and second major surfaces; a first conductivelayer disposed adjacent to the first major surface; a second conductivelayer disposed adjacent to the second major surface; and a firstcarbon-based layer and a second carbon-based layer each comprisingactivated carbon, carbon black and a binder disposed adjacent torespective ones of the first and second conductive layers, wherein thebinder comprises a fluoro-polymer having from 40 to 70 wt. % fluorineand a molecular weight of at least 500,000, and the first and secondconductive layers each have an organic content of less than 1 wt. %. 2.The carbon-based electrode according to claim 1, wherein thefluoro-polymer includes 50 to 70 wt. % fluorine.
 3. The carbon-basedelectrode according to claim 1, wherein the fluoro-polymer has amolecular weight of at least 800,000.
 4. The carbon-based electrodeaccording to claim 1, wherein the fluoro-polymer is polyvinylidenefluoride.
 5. The carbon-based electrode according to claim 1, whereinthe first and second conductive layers each have an organic content ofless than 0.5 wt. %.
 6. The carbon-based electrode according to claim 1,wherein the first and second conductive layers are free of organiccontent.
 7. The carbon-based electrode according to claim 1, wherein thefirst and second conductive layers comprise thermally-grown carbon. 8.The carbon-based electrode according to claim 1, wherein the first andsecond carbon-based layers have a thickness of from 20 to 500micrometers.
 9. The carbon-based electrode according to claim 1, whereinthe activated carbon has an average particle size of less than 20microns.
 10. The carbon-based electrode according to claim 1, whereinthe carbon-based layers include 75-90 wt. % activated carbon, 5-10 wt. %carbon black, and 5-15 wt. % binder.
 11. A method of forming acarbon-based electrode, comprising: forming a slurry including activatedcarbon particles, carbon black particles and binder; coating the slurryonto at least one major surface of a substrate to form a thin film; anddrying the thin film to form a carbon mat, wherein the binder comprisesa fluoro-polymer having from 40 to 70 wt. % fluorine and a molecularweight of at least 500,000.
 12. The method according to claim 11,wherein the coating comprises slot coating.
 13. The method according toclaim 11, wherein the thin film is formed over both major surfaces ofthe substrate.
 14. The method according to claim 11, wherein thesubstrate comprises a current collector having opposing first and secondmajor surfaces, a first conductive layer comprising thermally-growncarbon formed over the first major surface, a second conductive layercomprising thermally-grown carbon formed over the second major surface,and the slurry is coated onto each respective thermally-grown carbonlayer.
 15. The method according to claim 14, wherein the first andsecond conductive layers each have an organic content of less than 0.5wt. %.
 16. The method according to claim 11, further comprisinglaminating the carbon mat onto the substrate.
 17. The method accordingto claim 11, further comprising applying an alkali metalnapthalenide-based solution to the substrate prior to the coating,wherein the alkali metal is selected from the group consisting oflithium, sodium, and potassium.
 18. The method according to claim 17,wherein the solution comprises 1 to 10% wt. % sodium napthalenide. 19.An energy storage device comprising a first carbon-based electrode and asecond carbon-based electrode arranged within a casing, wherein eachcarbon-based electrode includes: a current collector having opposingfirst and second major surfaces; a first conductive layer disposedadjacent to the first major surface; a second conductive layer disposedadjacent to the second major surface; and a first carbon-based layer anda second carbon-based layer each comprising activated carbon, carbonblack and binder disposed adjacent to respective ones of the first andsecond conductive layers, wherein the binder comprises polyvinylidenefluoride having a molecular weight of at least 500,000.
 20. The deviceaccording to claim 19, wherein the first conductive layer and the secondconductive layer comprise conductive carbon.
 21. The device according toclaim 19, wherein the first conductive layer and the second conductivelayer are free of a binder material.
 22. The device according to claim19, wherein the device is an electrochemical double layer capacitor.